The sun's light and heat can cause considerable damage to fruit and vegetable crops. One type of damage is sunburn, the visible damage that begins on the fruit surface. A different type of damage, heat stress, can also cause significant loss. Heat stress is often less noticeable than sunburn, but plants undergoing heat stress respond by shutting down the photosynthetic process. When this occurs in late spring, plants can drop some of their fruit load. Heat stress also manifests itself as reduced foliar flushes, especially in young trees. While sunburn and heat stress are different, they both usually result from excessive exposure to infrared light from the sun and can be equally devastating in reducing crop quality and yield.
Sunburn has been identified to cause multi-million dollar crop losses. Apple producers have reported that sunburned damaged fruit is normally the major source of cullage and sunburn has damaged up to 50% of a harvested crop; an extremely important problem for the industry. In orchards for example, the shift to high-density plantings on size-controlling (dwarfing) rootstocks has resulted in smaller trees with less foliage to protect the fruit from solar radiation. Also, many new cultivars, e.g., “Fuji”, “Granny Smith”, “Jonagold”, “Gala”, and “Braeburn” are susceptible to sunburn. Consequently, the incidence of sunburn crop losses have increased as the shift to these new cultivars has occurred. It is being predicted that sunburn damage will continue to increase in the coming years as a result of the depletion of the stratospheric ozone layer leading to an increase in the ultraviolet-B radiation reaching the earth's surface and global warming.
Many variables play a role in the amount of sunburn damage that occurs in fruit crop such as meteorological elements, e.g., air temperature, relative humidity, the wind speed, direction and turbulence; the variety and physiological condition of the plants which influence the tree vigor, fruit size, transpiration efficiency, solar absorption, interception of solar energy, temperature tolerance, heat conductivity of the fruit, fruit coefficient of convective heat exchange, photo-stability, tolerance to ultraviolet radiation, degree of adaptation, and sensitization to the environment; the soil environmental condition, e.g., nutrient and water availability; air circulation; tree management, e.g., pruning practices; and so on. Thus, the amount of sunburn damage that a crop experiences is subject to a large range of complex, interacting factors.
Apple studies have distinguished three types of sunburn on fruit. One type is often noticed as a necrotic spot on the sun-exposed side of the fruit, and results from the thermal death of cells in the peel when the surface temperatures of the fruit reach about 126° F. Necrosis can begin in only 10 minutes at or above this temperature. Sunlight is not necessary for this condition to occur for it can result from high thermal exposure alone. Because of cell membrane damage, electrolyte leakage increases significantly in the skin or peel from fruit with necrosis.
The second type is called “sunburn browning”; a sub-lethal event that results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. Sunburn browning in apples occurs when the fruit surface temperatures reach from about 115° F. to 120° F. in the presence of sunlight. This threshold temperature required for sunburn browning is cultivar dependent. Electrolyte leakage from peel or skin that has sunburn browning damage does not appear to differ from the peel or skin of non-sunburned fruit. Thus, sunburned browning of the fruit appears to have little effect on membrane integrity although acceleration of the degradation of chlorophyll occurs. Of note, sunlight/solar irradiation is required for sunburn browning. Although sunburn browning sometimes appears to disappear as fruit matures, the disorder is not reversible. Instead, anthocyanins and other pigments sometimes mask these symptoms as the fruit color develops with maturity.
The third type of sunburn occurs on “non-acclimated” fruit that have been previously shaded and are suddenly exposed to full sunlight, e.g., after thinning or shifting of a branch as fruit load increases. This type of sunburn can occur at much lower air and fruit surface temperatures, such as in the fall. It does require light although it does not appear to require ultraviolet-B exposure. It has been observed in apples that this type of sunburn can occur when the air temperature is only 64° F., and fruit surface temperature is only 88° F. Initial damage is seen within 24 hours as bleaching or whitening of the sun-exposed skin surface. With continued exposure to sunlight, the bleached area turns brown.
In sunburned conditions, most of the cells under the epidermal layer of the fruit are damaged by low moisture content. The more severe form of this damage causes serious changes in the cuticle, and in the epidermal and sub-epidermal tissues. It is often noticed that in sun damaged fruit, the firmness of the flesh increases. This can be explained by the fact that the sunburned plant cells mainly perish because the cell walls thicken, and the tissues lose water and harden.
Sunburn and/or heat stress also can induce or enhance several skin and/or fruit disorders, such as lenticel marking (dark spots), sunburn scald, cracking/splitting, misshapen fruit, bitter pit (blotchiness), “Fuji stain”, and watercore. Watercore is a physiological disorder associated with internal moisture stress. High temperatures cause premature localized conversion of starch to sugar and pronounced sap leakage from cells, or an influx of sap into intercellular spaces. This causes a glassy appearance to appear on the surface of the fruit. Sunburned tissues can also serve as entrance points for fungi and other pathogens.
Numerous climate-ameliorating products have been used to reduce sunburn and/or heat stress on crop plants and their fruit. By the term “climate” is meant the environment above the surface of the soil in intimate contact with the surface of the plant and fruit and certain vegetables. Hereinafter, the term “fruit” will mean fruit and those vegetables, such as corn, tomatoes, and cucumbers that pre-harvest are exposed to the climate. The term “plant” means fruit-bearing plants. By the term “climate-ameliorating product” is meant the use of a device, water, or a chemical composition external to the plant or pre-harvest fruit that reduces the amount of heat and/or radiation impacting the surface of the plant or fruit above the surface of the soil. Among the most effective measures are evaporative cooling with water, and the use of sunburn protective coatings also known as particle film technology (PFT), bagging, reflective fabric, and shade netting.
Shade netting, mesh, or cloth can reduce air temperatures under the material by 4° F.-12° F. and increase the ratio of diffuse to direct sunlight on the fruit. The shade material also decreases transpiration rates resulting in increased mid-day water-use efficiency. This system can reduce sunburn, but it is very expensive because it requires a trellis system to hold up the shading material as well as the capability to apply it prior to harvest. Later, it must be removed for normal seasonal tree/vine growth. Excessive shading also negatively affects photosynthesis that, among other effects, will cause problems with flower bloom initiation early in the season and may affect yields. Growers have expressed concern about the deleterious effects that the shading can have on fruit trees, fruit bearing vines, and the soil. The protective mesh often interferes with the full, normal color development of fruit. Uniform shade can also cause an undesirable alteration in the growth habits of trees or vines and can reduce fruit production.
The use of reflective fabric, such as metalized surface plastics, white plastics, and foil materials on the ground of an orchard or vineyard can result in an increase in fruit size and yield with a concomitant reduction in fruit sunburn damage. Also, the fruit is often of a more consistent quality, i.e., of increased color, firmer, size uniformity, and sweetness. However, as with the shade netting, the labor and material costs are a limiting and often deciding factor with regard to the desirability or even feasibility of using this approach.
Evaporative cooling is one of the most efficient ways to reduce fruit surface temperature. The application of low volumes of water, cools the fruit through evaporative cooling of the surrounding air. When water is applied, it also evaporates from the fruit surface and cools the fruit. In addition, it hydrocools the fruit by carrying heat away in runoff. Such a system requires very high quality water with low salt content to prevent residues from being left on the fruit surface and extremely careful management to produce the desired cooling effect without increasing fireblight, apple scab, or root rotting diseases. This water aspersion option often requires an expensive installation; constant water spraying removes agrochemical products previously applied over the trees; and the increase in environmental humidity favors the development of diseases and weeds, affecting as a whole the phytosanitary status of the orchard. Application rates of 40-70 gallons per minute per acre are usually needed to cool fruit on the hottest days.
Although evaporative cooling, as noted above, is an effective means of reducing fruit temperatures and thus fruit damage, the demand for water in agriculture is in competition with urban, industrial, and recreational water demands. As a result, in addition to the expense associated with obtaining high quality water for such an evaporative cooling system, there are currently serious concerns over limited water availability in agricultural production.
Bagging fruit is normally used to control insect and disease pests but can be used to reduce sun damage on fruit. Each fruit or fruit bunch is usually covered with a double bag early in the season as the fruit begins to enlarge. Approximately 3 weeks prior to harvest, the outer bag is removed, exposing a transparent bag that protects the fruit but allows light penetration and good color development. This inner bag is removed just before harvest. This method is extremely expensive but can potentially increase profits if used on specialty fruits.
With respect to sunburn protective, particle film technology, ideally such a protective coating would have at least the following characteristics: (1) be composed of a chemically inert material, (2) spread and create a uniform film, (3) the film be porous so that it does not interfere with gas exchange from the leaf, (4) transmit photosynthetically active radiation but exclude ultraviolet and infrared damaging radiation to some degree, (5) reduce insect/pathogen behavior on the plant, and (6) be easily removed from the harvested fresh-market fruit. The industry is still searching for this perfect protective coating.
Historically, “whitewash” reflective, particle film technology (PFT), has been used to reduce heat stress. These reflective materials (unlike polymer film antitranspirants that physically block the stomates) have antitranspirant plant properties because they lower the leaf temperatures by increasing reflection of infrared radiation. The lowered leaf temperatures reduce the vapor pressure gradient between the leaf and the air, which is the driving force behind the transpiration; thus reducing the transpiration. It is critical that any product sprayed on a plant not interfere with the exchange of carbon dioxide through the stomates, otherwise primary productivity will be reduced. (Some antitranspirants increase stomatal closure to maintain high plant turgor and rigidity by reducing transpiration, but obstructing stomates can also reduce photosynthesis.)
Various commercial, reflective, particle film technology products are currently available that offer sunburn and/or heat stress protection. One of these products, Raynox®, a registered product of FruitGard LLC and manufactured by Pace International, is based on UV-absorbing plant waxes (carnauba wax). This wax deteriorates with solar radiation as time goes by, and as a result, it is usually necessary to apply the product many times to achieve a desirable result (Good Fruit Grower Mag., “Sunburn-reducing Films Compared” G. Warner, 2007).
Another product, Surround®, is described as a PFT product that is applied over the plant and fruit to screen UV and general solar radiation. This product is made from kaolin (mineral clay) as a suspension of finely divided white clay and it not only reflects incident radiation of the fruit but also radiation that reaches the plant leaves. This obviously decreases the efficiency of the photosynthetic processes of the plant, which affects not only its own growth and the growth of its fruit, but also the general health of the plant. This clay suspension can also limit plant transpiration because of stomatal blockage or occlusion. All of these effects cause increased stress to various biosynthetic pathways in the plant and/or a decrease of fruit quality or amount, i.e., their color and size. Also, fruit growth can cause the clay layer to break, thus losing a part of its protective ability, and requiring many applications during fruit growth and ripening. Kaolin formulations are also reported to suffer from substantial application problems such as excessive foaming and “globbing” in spray tanks. Kaolin powders are easily washed off by rain, further necessitating multiple applications or the use of stickers in order to maintain beneficial effects (NY Fruit Quarterly, Vol. 10, No. 1, Spring 2002).
Sunshield®, sold by Agrohytec, is taught to be applied over trees, vines and fruit to filter UV radiation and is described as a biodegradable polymeric protein micro-layer. The mode of action is similar to that of the carnauba wax products.
A recent product, Kool-Kore®, a trademark of OMRI Ltd, Pasco, Wash., is based on a composition of amorphous silica and surfactants, and is said to have a principle action similar to that of the kaolin-based compositions.
Another product is defined as a proprietary mixture of calcium carbonate and clay. These mixtures are reflecting clays and thus operate using the same mode of action.
Growers have disclosed several problems relating to the use of PFT material in general in that the compositions often plug spray nozzles, and require multiple applications that are very labor intensive. Furthermore, these coatings on the fruit after harvest are often difficult to wash off, off-times requiring additional washing and scrubbing procedures. In some cases, growers have been required to install additional wash tanks, use cleaning detergents, change brush lengths and/or shapes, and increase water pressure in the rinses. Even with these extra steps, it has proven difficult to remove coating material from the skin in the well around the stems. This is not a serious concern with fruit destined for further processing such as apples being sold for apple sauce or canning or wine grapes, however it is a major issue for table grapes and other fruit to be sold in the fresh fruit markets.
In summary, there currently is a lack of adequate means to prevent sunburn and/or heat stress in fruit crops. Efforts to date have been directed toward modifying the climate to which the fruit and the fruit plant are exposed. This approach, in addition to being a major expense to the grower as a result of the need for additional installation of equipment and/or being labor intensive, has proven to be fraught with difficulties and less than satisfactory results. Thus there is a strong need in the agricultural and viticultural industries for an inexpensive and effective means to mitigate fruit environmental damage, especially sunburn and/or heat stress damage, that is long lasting; is relatively amenable to easy application by growers; and does not require additional, expensive steps for protective film removal.
The instant invention provides the above-enumerated advantages and, in addition, enhances plant vigor, especially plant growth, fruit yield, density, color, and quality.